合并两个排序链表

将两个排序链表合并为一个新的排序链表

样例

给出 1->3->8->11->15->null，2->null， 返回 1->2->3->8->11->15->null。

解题思路：新开一个新链表，比较两个链表元素的大小，一次向新链表中添加即可。

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param ListNode l1 is the head of the linked list
     * @param ListNode l2 is the head of the linked list
     * @return: ListNode head of linked list
     */
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // write your code here
        ListNode result = new ListNode(0);
        ListNode head = result;
        while(l1!=null && l2 !=null){
            if(l1.val <= l2.val){
                result.next = l1;
                l1 = l1.next;
            }else {
                result.next  = l2;
                l2 = l2.next;
            }
            result = result.next;
        }
        if(l1 == null )   result.next = l2;
        if(l2 == null )   result.next = l1;
        
        return head.next;
    }
}